As noted in Section 3.1, both the \(\) 1640 and \(\) 4686 lines of He ii suffer from severe blending with other lines. However, one can use the strong, rapid variability of He ii to reduce the effects of contamination of the He ii \(\) 1640 and \(\) 4686 lines by other spectral features. Because the ionization potentials of oxygen are only slightly higher the those of carbon for the same stages of ionization, O iii] \(\) 1663, which blends with He ii \(\) 1640, is produced in approximately the same region as C iii] \(\) 1909. Photoionization models show that C iii] \(\) 1909 is produced at column densities at order of magnitude higher than He ii. The self-shielding model of [1]} hence predicts that O iii] will be emitted at a radius an order of magnitude larger than He ii. This is strongly supported by reverberation-mapping. For NGC 5548, [2]} get a C iii] \(\) 1909 lag of 34 days behind the continuum whilst [3]} get a He ii lag of \(< 2\)  days. O iii] \(\) 1663 therefore cannot follow rapid variability of the ionizing continuum the way He ii \(\) 1640 can and does. This is true a fortiori for the optical Fe ii emission blended with He ii \(\) 4686 because Fe ii arises in the lowest ionization part of the BLR with an effective radius twice that of H\(\)  [4]}. For NGC 5548, Fe ii variability will lag the continuum variability by \(\sim 40\)  days. Given this much slower response of contaminating lines in the UV and optical, we propose two methods that exploit the stronger variability of He ii for estimating the contamination-free He ii \(\) 1640/\(\) 4686 ratio.
